The present invention relates to a line head in which a plurality of light-emitting elements are arranged in a line, which enables to equalize the amount of emitted light from the respective light-emitting elements, and an image forming device using the line head.
Also, the present invention relates to a line head in which a plurality of light-emitting elements are arranged in a line, and which enables to have evenness in quantity of emitted light, irregardless of turning-on patterns of the respective light-emitting elements, and an image forming device using the line head.
Further, the present invention relates to a line head which is configured so as to remove the variations in amount of light emitted from a plurality of light-emitting elements and to reduce the size of the line head when the plurality of light-emitting elements are arranged in one line, and to an image forming device using the line head.
An image forming device including a line head which has a plurality of light-emitting elements arranged in a line is developed. The line head is used as an exposing unit. JP-A-6-64229 describes that EL (electroluminescence) elements for one line are arranged in an optical printer head, and grayscale data corresponding to the respective EL elements is stored for every EL element. Further, in JP-A-11-198433, a printer head having a plurality of LED chips arranged in a line which can improve unevenness of light-emitting characteristic in a scanning line direction is described.
FIG. 34 is a diagram illustrating schematically an example of a wiring line configuration of a related organic EL element. In FIG. 34, a plurality of organic EL elements Ea are arranged in a line head 10 in a scanning line direction, to thereby form one light-emitting element line 1. Reference numerals 2 and 3 denote first and second power supply lines formed with thin film wiring lines, and reference numerals 6 and 7 denote feeding points. The feeding point 6 is provided at a power supply (VDD) side, and the feeding point 7 is provided at a ground (GND) side. Further, a reference numeral A denotes an anode electrode of the organic EL element Ea, and a reference numeral K denotes a cathode electrode thereof.
A reference numeral Tr2 denotes a drive transistor which is formed on the same substrate as the organic EL element Ea. A reference numeral D is a drain of the drive transistor Tr2 which is connected to the power supply line 2. A reference numeral G denotes a gate of the drive transistor Tr2 , and a reference numeral S denotes a source of the drive transistor Tr2 which is connected to the anode electrode A of the organic EL elements Ea. Moreover, though not shown, the gate G is connected to a source of a control transistor Tr1 via a wiring line Ga. An external circuit 12 which extends to the lengthwise direction at a short side of a housing 11 of the line head is provided in the housing 11, and is connected to feeding points 6 and 7 by a feeding cable. Furthermore, a control signal line for controlling a control transistor (not shown) or a drive transistor which is formed in the light-emitting element line 1 is wired from the external circuit 12.
FIG. 35 is a circuit diagram of FIG. 34, and the same elements as those of FIG. 34 are represented by the same reference numerals. As shown in FIG. 35, in the control transistor Tr1, a signal line 4 of a gate and a signal line 5 of a drain are provided. Further, as described above, the drain of the drive transistor Tr2 is connected to the first power supply line 2, and to the gate thereof, the source of the control transistor Tr1 is connected. The respective organic EL elements arranged in the light-emitting element line 1 is connected between the first power supply line 2 to be connected to the feeding point 6 of the power supply (VDD) side, and the second power supply line 3 to be connected to the feeding point 7 of the ground (GND) side.
A light-emitting element using the organic EL element is a current-driven element, current flowing in the power supply line (VDD side) of a drain side of the drive transistor Tr2 and in the power supply line (GND side) of a cathode (cathode electrode) side of the light-emitting element increases or decreases according to the degree of light-emission of the light-emitting element. Here, the first and second power supply lines are formed with the thin film wiring lines, and the resistance values of both ends of each of the power supply lines are different from each other according to the size of the printer head. For example, the resistance values are in an order of several ohms to tens of ohms.
Further, when all the light-emitting elements are turned-on, the current of each of the light-emitting elements is in an order of at least ten mA, and voltages to be applied to the respective light-emitting elements reach tens of millivolts to hundred millivolts. Here, in the case in which the organic EL element is used as the light-emitting element, it is well-known that due to a slight difference of the applied voltages, the current changes, that is, the amount of emitted light of the respective light-emitting elements change greatly. Therefore, there may be a case in which the amount of emitted light change largely, in particular, according to distances from the respective light-emitting elements to the feeding points.
FIG. 36 is a simplified circuit diagram of FIG. 34. In FIG. 36, a left end organic EL element Ea is represented by a reference numeral E1 and a right end organic EL element Ea is represented by a reference numeral En. Reference numerals R and nR denote wiring line resistances. The reference numeral R denotes the wiring line resistance between the feeding points 6 and 7 and the left end organic EL element E1, and the reference numeral nR denotes the wiring line resistance between the left end organic EL element E1 and the right end organic EL element En.
When a voltage and a current between the feeding points 6 and 7 are V and i, respectively, an applied voltage of the organic EL element E1 is Vp1, and an applied voltage of the organic EL element En is Vpn, the expressions of Vp1=V−4Ri and Vpn=V−4Ri−4nRi are satisfied. In such a manner, when the plurality of light-emitting elements are arranged in a line, and the respective light-emitting elements are connected between the first and second common power supply lines, the voltages to be applied to the respective light-emitting elements are different from each other according to the distance from the feeding point. In FIG. 36, a voltage difference in the light-emitting elements at both ends of the line becomes large, and then the amount of emitted light are different from each other. Since the life span of the light-emitting element is shortened as brightness increases, unevenness in the life spans of the light-emitting elements is caused. Further, if the amount of emitted light are different from each other, the lowering of printing quality caused.
Further, different currents flow into the respective light-emitting elements arranged in a line due to turning-on patterns. That is, a current from a power supply line flows into the light-emitting element to be turned on, and a current from a power supply line doesn't flow into the light-emitting element to be turned off. Therefore, a potential of the power supply line which applies a voltage to the light-emitting element at a position of the light-emitting element to be turned on, and a potential of the power supply line at a position to be turned off are different. In such a manner, due to the shapes of the turning-on patterns, the light-emitting elements to be turned on and the light-emitting elements to be turned off exist in the line, and thus a change in potential of the power supply line is caused. For this reason, unevenness in quantity of emitted light of the respective light-emitting elements is caused.
Therefore, in the example of FIG. 36, a difference between voltages to be applied to the light-emitting elements becomes large due to positions to be connected to the line, and further the voltages to be applied to the respective light-emitting elements change due to the turning-on patterns. Thus, unevenness in quantity of emitted light is caused. Since the life span of the light-emitting element is shortened as brightness increases, unevenness in life span of the light-emitting elements is caused. Further, if irregularity in quantity of emitted light exists, the lowering of display quality is caused. In the example of FIG. 36, a difference between the voltages applied to the light-emitting elements is caused by a position of the connecting position with regard to the power supply line 2 of the light-emitting elements and a turning light pattern of the light-emitting elements. Therefore, when a slightly voltage variation between the power supply lines 2 and 3 is occurred by the disturbance, an influence for an amount of the light emitting of the light-emitting elements becomes larger.
In order to solve the above problems, it is preferable to broaden the widths of the first and second power supply lines 2 and 3. In this case, however, the width of the light-emitting element becomes large, and then the size of a printer increases. Further, in the case that a substrate having the same size is used, there is a problem in that the number of the light-emitting elements to be manufactured decreases. As another solution, it is preferable to form a thick power supply line. However, since the light-emitting element is formed with a multi-layered thin film process, it is impossible to form the power supply line thicker than is necessary. At most, the thickness of the power supply line is limited to about hundreds of micrometers.
Further, if only the power supply line is thickened extremely, a step difference with other layers becomes large, and then separations or defects of the thin film layers are caused. In addition, the line head has a shape long in a main scanning direction with a narrow width in a sub-scanning direction. In such a manner, since the shape of the line head is extremely long and slender, a curve is caused by a difference in thermal expansion coefficient with the substrate (glass). Moreover, if the thin film is thick, the time for forming the film becomes longer, and the man-hour rises. That is, there are many problems to be caused intrinsically by the shape of the line head or the manufacture of the light-emitting element.
In the devices described in JP-A-6-64229 and JP-A-11-198433, as shown in FIG. 34, all the feeding points connected to the first and second power supply lines are provided at the same side of the line. Further, the voltages to be applied to the respective light-emitting elements change by turn-on patterns. For this reason, there is a problem in that various problems described above are not solved. Further, there is a problem in that since the external circuit is provided at a short side of the line head in the lengthwise direction, the size of the housing of the line head increase, and thus the space is insufficient.